The present invention relates to a method of producing a plastic injection molding, particularly molding strips for automotive vehicles, in which a plastic injection molding composition is injected with high injection pressure into an injection mold and it deposits on and bonds to individual regions on at least the front side of a support which is inserted into the mold cavity of the injection mold, in order to form a molded plastic layer on the support. The invention also relates to an apparatus for carrying out this method which comprises a plastic injection molding machine having an injection mold between whose mold parts the support can be clamped. The half of the mold which supports the front side of the support has at least one recess which forms a mold cavity for the molded layer.
Frequently, in plastic injection molded parts, and particularly molding strips for automotive vehicles, the support must have regions which are free of deposited plastic material, and these plastic material-free regions must be developed with clean edges. If a strip of alloy steel, possibly pre-shaped, is used as the support, as is frequently the case in connection with molding strips for automotive vehicles, a plastic material-free region here serves solely in order to impart a decorative effect to the molding strip. Molding strips often have plastic material-free longitudinal strips which extend over practically the entire length of the molding strip, are of constant width over their entire length, and are known as "bright regions". It is difficult to manufacture molding strips with clean linear development at the edges of the plastic layer which define the "bright regions". The dificulties are due, first of all, to the sealing function between the edges of the mold cavity and the adjoining support surfaces of the corresponding mold part of the injection mold, which function must be satisfied exclusively by the support which is pressed by the closing pressure of the injection mold against the resting surfaces of the mold parts. Secondly, as a rule, the bright regions are provided on arched or otherwise curved regions of the molding strip, and these are regions in which deviations in tolerance can hardly be avoided in practice. Third, with molding strips, in order to avoid visible points at which the injected material has entered the molds through runner channels, the injection molding composition is injected through a runner channel at one end of the strip into the injection mold. This requires very high injection pressure, which may be more than 1,000 bars, depending on the length of the strip and its cross section. In accordance with the traditional manner of manufacture, if a support is inserted into the lower part of the mold, and the injection mold is closed, and plastic injection molding composition is injected under high pressure into the mold cavity of the upper mold part, then the pressure of application of the support against the resting surface of the lower mold part is increased. With even minimum deviations in the support due to tolerances, there will be a deformation of the support and a gap will form between the top of the support and the adjacent resting surface of the upper mold part, at the edge regions of the mold cavity. However, since the injection molding composition tries to fill up even the smallest spaces, courses more or less unclean course of plastic material form the edges of the bright regions after the removal of the injection molded part from the mold. Since unclean edges in the bright regions of the molding strips are extremely undesirable, costly additional operations must be carried out. As a result, the surface of the bare bright region is frequently damaged, i.e. scratched.